Communication devices such as User Equipments (UEs) are also known as e.g. terminals, mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the user equipment. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Device-to-device (D2D) communications underlying a cellular infrastructure has been proposed as a means of taking advantage of the physical proximity of communicating devices, increasing resource utilization, and improving cellular coverage. Relative to the traditional cellular methods, there is a need to design new peer discovery methods, physical layer procedures, and radio resource management algorithms that help realize the potential advantages of D2D communications. Here the 3GPP Long Term Evolution system is used as a baseline for D2D design, review some of the key design challenges, and propose solution approaches that allow cellular devices and D2D pairs to share spectrum resources and thereby increase the spectrum and energy efficiency of traditional cellular networks. Simulation results illustrate the viability of the proposed design.
Device-to-device (D2D) communications in cellular spectrum supported by a cellular infrastructure holds the promise of three types of gains. The proximity of user equipments (UE) may allow for extremely high bit rates, low delays and low power consumption. The reuse gain implies that radio resources may be simultaneously used by cellular as well as D2D links, tightening the reuse factor even of a reuse-1 system. Finally, the hop gain refers to using a single link in the D2D mode rather than using both an uplink and a downlink resource when communicating via the access point in the cellular mode. Additionally, D2D communications may extend the cellular coverage and facilitate new types of wireless peer-to-peer services. However, D2D communications utilizing cellular spectrum poses new challenges, because relative to cellular communication scenarios, the system needs to cope with new interference situations. For example, in an orthogonal frequency division multiplexing (OFDM) system in which D2D communication links may reuse some of the OFDM time-frequency resources (physical resource blocks, PRB), intracell interference is no longer negligible. In addition, in multicell systems, new types of intercell interference situations have to be dealt with due to the undesired proximity of D2D and cellular transmitters and receivers. Interestingly, these new types of interference situations are intertwined with the duplexing scheme that the cellular network and the D2D link employ, and also depend on the spectrum bands and PRBs allocated to D2D links. For example, when a D2D link utilizes some of the cellular uplink PRBs, a transmitting cellular user equipment (UE) may cause much stronger interference to a receiving UE of a D2D pair in a neighbor cell than the interference caused to a radio base station in that same neighbor cell. Solution approaches to deal with this problem include power control, various interference avoiding multi-antenna transmission techniques that can be combined with proper mode selection which decides whether a D2D candidate pair should be communicating in D2D or in cellular mode and advanced (network) coding schemes. The key functions of D2D communications comprises peer discovery, physical layer procedures, such as synchronization and reference signal design, and various radio resource management functions including mode selection, scheduling, PRB allocation, power control, and intra- and intercell interference management. See “Design Aspects of Network Assisted Device-to-Device Communications” Gabor Fodor, Erik Dahlman, Gunnar Mildh, Stefan Parkvall, Norbert Reider, György Miklós and Zoltán Turányi, IEEE Communications Magazine March 2012.
In cellular network assisted Device-to-Device (D2D) communications, also referred to as D2D communications as a cellular underlay, user equipments in the vicinity of each other typically less than a few 10s of meters but sometimes up to a few hundred meters, can establish a direct radio link, also referred to as a D2D bearer. While user equipments communicate direct over the D2D bearer, they also maintain a cellular connection with their respective serving base station. In this way the cellular RAN can assist and supervise the user equipments in allocating time, frequency and code resources for the D2D bearer. Also, the cellular RAN controls mode selection, meaning that the cellular RAN decides whether the D2D pair should use the direct link or communication should take place via the base station. The RAN also sets the maximum power level that the D2D pair may use for the D2D bearer.
Thus the basic rationale for network assisted D2D communications is to take advantage of the short distances between user equipments, reuse cellular spectrum and at the same time to protect the cellular layer from potentially harmful interference caused by the D2D bearer.
Device discovery, also called neighbor discovery or peer discovery, is a procedure that allows devices in the vicinity of each other to detect one another. Existing solutions for device discovery assume that the devices operate in the same frequency band and/or that the devices are registered at the same cellular network operator.
Therefore, existing techniques do not support device discovery procedures for devices operating in the licensed spectrum bands of different cellular operators. As an example, using existing techniques, a device (Device A) operating in the licensed spectrum of Operator-A cannot discover another device (Device B) registered and operating in the network of Operator-B.
Thus, the problem to be solved is that devices served by different cellular operators cannot discover one another, even if operating in the physical proximity of each other.
This problem should be solved in such a fashion that can be acceptable by end users, operators and regulatory bodies.
Device discovery is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth, several variants of the IEEE 802.11 standards suite, such as WiFi Direct. The key technique used by these standards is to use specially designed beacon signals that devices can broadcast so that nearby devices can detect the proximity of such beacon broadcasting devices.
Recently, device-to-device communications as an underlay to cellular networks have been proposed as a mean to take advantage of the proximity of communicating devices and at the same time to allow devices to enjoy a controlled interference environment. Various device discovery techniques applicable for devices in cellular spectrum have also been proposed recently. These techniques make use of various forms of network assistance, such as obtaining synchronization, peer discovery resources (PDR) or tuning other parameters of the discovery process.
For the situation when two devices are connected to the same NW, this allows for a very efficient way for devices to discover one another since the NW then tells the different devices when to send their respective beacon and when to listen for certain beacons corresponding to other devices in the vicinity.
Although device discovery for ad hoc networking type of technologies (Bluetooth, WiFi Direct, is a relatively mature technology, only very few existing techniques have been proposed and built for devices operating in cellular spectrum. Therefore, the vast majority of existing (i.e. described, disclosed or actually built) solutions assume D2D device discovery operation in unlicensed spectrum. In this case, the implicit assumption of device discovery has been that all devices transmit and listen to beacon or reference signals within the same frequency bands.
There are also a few disclosed solutions proposed for device discovery for devices operating in cellular spectrum. This type of D2D communications in general and D2D discovery in particular have been termed “D2D communications as an underlay of cellular networks”. The basic assumption for this type of D2D communications has been similar to that of the ad-hoc type of communication, but now assuming that all devices operate in the same, e.g. licensed, spectrum bands.